Diatoms

Abstract

Diatoms are single‐celled autotrophic organisms with highly ornate siliceous walls. They account for more than 20% of the
world's primary production, are responsible for much of the petroleum humans use, and deposits of their shells are mined for
numerous uses. The most taxonomically diverse groups of photosynthetic protists, diatoms are of ecological importance in nearly
every freshwater and marine habitat. Their closest relatives are a poorly known group of microflagellates, some silicified
and some not. Efforts to reconstruct diatom phylogeny are hampered by the fact that their true diversity and that of their
closest relatives remains unknown. Genomic studies are creating tremendous new opportunities for study of this important group.
Perhaps as much as a third of the diatom nuclear genome is now thought to be recently laterally transferred from bacteria.
Diatom adaptation to such a wide range of environments may be a result of the resulting novel gene combinations.

Key Concepts:

Diatoms are a highly diverse group of protists, with perhaps more than 200 000 species, most of which remain undescribed.

There are four major structural groups, radial centrics, polar centrics, araphid pennates and raphid pennates.

Among the four major structural groups of diatoms, only the raphid pennates are robustly supported as monophyletic.

Diatoms dominate the world's oceans, lakes and streams, conducting more than 20% of the world's photosynthesis.

Fossil diatoms have significant economic importance as stratigraphic indicators for mineral exploration, particularly petroleum
and as a source of diatomite, which is used in numerous industrial applications.

Diatoms make a diverse number of nanoscale siliceous structures, an order of magnitude smaller than that presently attainable
by human technology.

Understanding how diatoms make their shells may lead to breakthroughs in nanotechnology.

Nearly a third of the diatom nuclear genome appears to be the result of relatively recent horizontal gene transfer from bacteria.

Horizontal gene transfer may play a role in the apparent rapid adaptation and diversification of diatoms.

Generalised diatom life history. OE, original epivalve; OH, original hypovalve.

Figure 2.

Scanning electron micrograph of a plankton diatom assemblage from Lake Michigan, USA. The diatom dominating the centre is
a centric diatom (genus Stephanodiscus). Numerous araphid pennate diatoms (genus Asterionella) are also visible, as is another centric diatom (genus Aulacoseira) at the bottom. For the most part, only valves are visible here, as the cell walls were disrupted by acid cleaning, a common
preparation technique for better viewing in the scanning electron micrograph. For scale, the Stephanodiscus cell is approximately 50 μm in diameter. Species of this genus almost always have radiating ribs ending in a spine, as visible
here. In contrast to Stephanodiscus, the cells of Aulacoseira are typically much taller than they are wide, and so appear as tubes rather than discs. The valves of Asterionella are long and thin, but end in a slightly pinched and then expanded tip. All these variations and much more occur in the minute
valves of diatoms (see Round et al., ).

Figure 3.

Phylogenetic relationships of large structurally defined groups of diatoms based on maximum likelihood analysis of SSU, rbcL and psbC genes (see Theriot et al., for details). Representative taxa of larger clades are indicated by triangles. Long tubular diatoms such as Aulacoseira (Figure ) form a grade at the base of the diatom tree. Most discoid radial centrics are found in a single clade. But these are not
the only places on the tree where such shapes occur. Polar centrics include diatoms of a wide variety of shapes and forms,
from star‐shaped cells, to cells with long tubular shape (such as Stictocyclus), and cells with a flat discoid shape such as Stephanodiscus (Figure ).

Figure 4.

Stages of periphyton development on glass‐slide substrates from Pawnee Reservoir, Nebraska. (a) Typical development after
2 weeks of submersion; diatoms and other microalgae are attached directly to the glass. (b) After 5 weeks, an upper story
of diatoms on long stalks has developed, completely overgrowing all lower canopies. Bar, 100 μm. Photographs courtesy of Kyle Hoagland.

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